1. Field of the Invention
This invention relates to electronic systems, and more particularly, to DC-DC converters for power distribution systems.
2. Description of the Related Art
Efficient DC-DC converters are used to generate high currents at low voltages for many modern computer systems. Typically, these DC-DC converters are place on the same printed circuit board (PCB) as those circuits to which they are providing power. However, placing the DC-DC converter on the PCB may be undesirable from a reliability point of view. It would be more desirable from a reliability standpoint to place the converter on a different FRU (field replaceable unit), wherein the PCB and the DC-DC converter could be replaced independent of one another.
In a power distribution system, inductance (and more particularly, inductive impedance) between the DC-DC converter (or a voltage regulator) and the load is undesirable. Such inductive impedance can lead to higher levels of noise in the power distribution system, and this in turn can cause the release of electromagnetic energy that results in EMI (electromagnetic interference) or EMC (electromagnetic coupling). Power distribution system noise is a product of its impedance and its current. Thus, it is especially important to keep impedance at a minimum in today's computer systems, which often draw a high amount of current.
Inductive impedance in a power distribution system may be reduced by the use of decoupling capacitors. However, ESL, or Equivalent Series Inductance (sometimes referred to as mounted inductance) may reduce the effect of providing decoupling capacitors. The ESL of a capacitor is that inductance associated with its mounting on a printed circuit board (PCB) or other circuit carrier. A capacitor's ESL is a function of both the internal capacitor characteristics and its mounting on the PCB (which includes various factors such as mounting geometry of the capacitor).
In placing a DC-DC converter on a different FRU than the circuit to which power is being provided to, a significant amount of inductance may be introduced in the connection between the two, including inductance on the FRU itself. To counter the FRU inductance (and thereby reduce the impedance), a relatively large amount of capacitance may be implemented near the DC-DC converter. The capacitance may ensure that the output of the DC-DC converter is stable and smooth. However, this does not counter the inductance in the connectors and bus bars between the DC-DC converter and the load. As such, a significant amount of capacitance is also required at the load in order to ensure a low-impedance current path from the DC-DC converter.
Implementing a large amount of capacitance implemented on the DC-DC converter and the load board may result in additional poles and zeros in the transfer function that describes the relationship of the power input to the power output. In some cases, this transfer function may be a fourth-order equation, and as such, stabilizing the power distribution system becomes more difficult. Furthermore, adding a large amount of capacitance to both the DC-DC converter and the load board can drive up the cost of both.